Aluminum-tin alloy



June 1, 1965 M. J. PRYOR ETAI.

ALUMINUM-TIN ALLOY 3 Sheets-Sheet 3 Filed Feb. 5, 1962 w 0 3 m o mmmznz 0.7 0.8 0.9 l.0 l.l -l.2 l.3 l.4 COUPLE POTENTIAL (VOLTS) INVENTORSZ MICHAEL J. PRYOR DOUGLAS s. KEIR 3 B PHILIP R. SPERRY Y ATT NEY United States Patent 0 3,186,836 ALlu-TIN ALLOY Michael J. Pryor, Hamden, Philip R. Sperry, North Haven,

and Douglas S. Keir, Hamden, Conn., assignors to Olin Mathieson Chemical Corporation, a corporation of Virginia Filed Feb. 5, 1962, Ser. No. 171,114 11 Claims. (Cl. 75138) This application is a continuation-in-part of the application Serial No. 60,166, filed October 3, 1960.

The present invention relates to aluminum alloys having exceptional galvanic properties. More particularly, it relates to aluminum alloys from which high galvanic currents can be obtained in combination with high galvanic efiiciency.

As was pointed out in the earlier application, a problem which has been recognized for a long time in the art of use of metals in galvanic applications is that of providing a low cost, commercially available metal in a form which has desirable galvanic properties for many uses. Numerous metals and alloys have been proposed for these purposes and a number of metal compositions have been found to have galvanic properties which make them suitable for particular applications. The degree of utility which is found for a particular metal depends, of course, on the combination of galvanic properties required for a specific use and on how well this combination of properties can be met by lower cost metals and alloys.

While the earlier application referred to above provided a highly versatile solution to the problem of developing a variety of aluminum compositions containing tin having unique galvanic properties and the methods of preparing and using such compositions, there are distinct advantages to be obtained from having the unique galvanic aluminum compositions available at lower cost.

Alloys containing high percentages of tin in aluminum have been known and studied for many years and various values have been reported for the solubility of tin in aluminum and for the solubility of aluminum in tin. However, none of the known alloys of tin and aluminum which have been known have been shown to exhibit a combination of galvanic properties such as those produced pursuant to the present methods.

Aluminum compositions containing large amounts of tin have been used, for example, as hearing alloys. Other aluminum alloy compositions have been used in which small tin additions have been utilized as for example in Al-Cu base alloys to modify rates of age hardening. 7

Accordingly, one object of the present invention is to provide an alloy of aluminum which exhibits improved.

galvanic behavior.

Another object is to provide a method of imparting improved galvanic properties to aluminum.

A further object is to provide a method of controlling the galvanic properties of alloys of aluminum and tin.

Still another object is to provide an alloy having a combination of galvanic properties which are variable over a wide range not currently attainable in commercially available alloys.

A further object is to provide an alloy of aluminum which has improved galvanic properties and which can be prepared from aluminum alloy of commercial purity.

Other objects will be in part apparent and in part pointed out in the description which follows.

In one of its broader aspects the objects of the invention are achieved by providing a galvanic aluminum alloy containing between about 0.1 and 0.5 percent tin in a condition to render said alloy galvanically active and containing no more than about 1 percent of elements which tend to mask said galvanic properties.

Patented June 1, 1965 In one of its narrower aspects related to the production of high galvanic currents at high anodic efliciency, these objects are obtained by providing an aluminum base metal composition containing less than 0.05 percent silicon, containing less than O.1 percent iron and containing between 0.1 and 0.3 percent tin where the composition is in the homogenized condition.

The practice of these and other aspects of the invention will be understood more fully from the description which follows. In this description reference will be made to the accompanying drawings in which:

FlGURE 1 is a graph illustrating the relationship between certain galvanic properties of aluminum-tin compositions and the silicon contents of the compositions.

FIGURE 2 is a similar graph showing the same relationship for aluminum-tin compositions of increasing iron content.

FIGURE 3 is a graph illustrating the relationship between cell potential of a galvanic cell containing aluminum and current.

It has now been discovered that high galvanic currents coupled with high anodic efficiencies can be obtained at greatly reduced cost where this cost is relative to the cost of similar currents obtained from galvanic oxidation of metal compositions known heretofore. The cost of electrical energy produced from such galvanic oxidation is so low in fact that auxiliary power sources which depend on the galvanic oxidation of aluminum composition of the present invention are rendered economically competitive with other power sources, such as internal combustion engines, for certain applications. The achievement of such economic production of electrical power pursuant to this invention is dependent on two principal factors.

The first isthat it is possible to retain many of the highly desirable galvanic oxidation properties of the compositions taught in the parent application Where certain ternary alloying elements are present in the higher purity aluminum base composition of the parent application, and even to enhance certain desirable properties of the compositions by such additions. The attainment of high galvanic currents on a sustained basis is due in part to the capability of the composition to develop surface layers on oxidation of any portion thereof, which surface layers have an excess of n-type defects in a concentration etlective to substantially increase the conductivity thereof, i.e., to increase the conductivity of such layers by more than 100 percent.

The attainment of high galvanic current on a sustained basis and at high anodic efliciency is also due in part to the presence within the compositions of secondary cathodes in a size, form, distribution, and concentration which meets the requirements more fully set out below.

A second factor on which the attainment of low-cost power from the galvanic oxidation of aluminum depends is the very striking response of the alloy compositions to post-casting thermal treatment with regard to the effect of such treatment on control of the galvanic properties of the compositions. The post-casting thermal treatment may be used to control the concentration of the tin in metastable solid solution within the aluminum alloy. A principal advantage of the post-casting treatment is in maximizing the amount of tin which is in metastable solid solution, inasmuch as the higher concentrations of tin in solid solution have been found to be essential to the attainment of higher galvanic currents from the composition. As will be brought out more fully below the attainment of high galvanic currents in aluminum base compositions containing tin, wherein the aluminum is of lower purity, depends to a large extent on maximizing the concentration of tin in solid solution through the homogenization treatment.

It is also feasible to control certain properties of the a) V compositions by modification of the casting procedures. For example, it is possible to increase the size of certain insoluble intimetallic compounds, such as FeAl by reducing the rate of cooling of the melt during casting to retard solidification, and to allow the FeAl particles to grow to a desired size.

Each of these factors will be considered in turn with regard to its effect on the provision'of aluminum alloy compositions having desired galvanic properties at low cost. a

With regard first to the purity of the aluminum base of the galvanic alloy the impurities. found most commonly in commercial grades of aluminum tendto mask the remarkable display of high galvanic current now found to be possible in aluminum base alloys containing galvanically activating amounts of tin. In order to provide a lowcost galvanic aluminum therefore with a high galvanic current and a high current efliciency pursuant to this invention, it is necessary not only to provide tin additive in the needed amount and form, but it is necessary also to restrict the common impurity level to predetermined low values.

With regard to one of the most common impurities normally found in aluminum-base alloys, namely. silicon, it has been found that aluminum-tin alloys containing from 0.1 to 0.3 percent tin and containingup to 0.05 percent silicon will yield high galvanic currents if the alloy is in the homogenized condition. As illustrated graphically in FIGURE 1, where higher concentrations of silicon are present, even where tin is present in concentrations which otherwise yield high galvanic currents,

and where this composition is in the homogenized condition, high galvaniccurrents above 500 coulombs flowing in a reference galvanic cell in 48 hours, are not produced.

However, useful galvanic currents are produced from such compositions containing concentrations of silicon in excess of 0.05 percent as is evident from FIGURE 1 and as will be explained more fully below.

Regarding now the concentration of the iron impurity commonly found in aluminum, it has been discovered that where the iron content of homogenized aluminum-tin compositions containing 0.1 to 0.3 percent tin, does not exceed 0.1 percent, high galvanic currents can be obtained at high anodic efficiency, i.e., above about 50 percent efiiciency. As illustrated in FIGURE 2, at values of iron concentration substantially in excess of 0.1 percent, the values obtained for certain other galvanic properties are reduced below the desirable high values attainable at lower iron concentrations. J

Thus, it is possible to retain many of the desirable galvanic and other properties of compositions containing tin in high purity aluminum as described in the copending application Serial No. 60,166, although the tin is present in the homogenized form in a low-cost aluminum base containing up to 0.1 percent iron, up to 0.05 percent silicon, and concentrations of other impurities inelfective to mask these desirable galvanic properties.

It may be advantageous to raise the tin concentration from the preferred range of 0.12 to 0.15 percent for the high purity aluminum base to concentrations of 0.2 percent and higher as the concentrations of other constituents increases. For example, when the silicon content is'about 0.05 percent, the tin concentration is preferably raised to about 0.2 percent. e

For purposes of clarity, .and easy reference, a numberof terms in the foregoing and following description are defined herein;

For one such definition reference is made to the accom- .panying drawings in which FIGURE 3 is a graph utilizing the data from Table 1, below, illustrating the relationship found to exist between the level of galvanic current produced from a reference galvanic cell' and the potential which is found to exist between the electrodes of the cell. In FIGURE 3 the number of coulombs flowing in 48 hours in a reference galvanic cell is the ordinate and the closed circuit potential, on the hydrogen scale, found to be developed in the reference cell is abscissa. In general, it will be observed from FIGURE 3 that there is a remarkable increase in the level of current which is produced for a unit increase in the potentialexisting. between the electrodes when the potential is more negative than 0.9 volts, as compared to the increase of galvanic current produced for a unit increase in the voltage where this voltage is less negative than 0.9 volts.

Because-of the very sharp change in the level of galvanic current which is produced with each unit increase in voltage at voltages more negative than -0.9 in the reference cell, aluminum compositions having the capability of producing current continuously in the reference cell at a potential more negative than 0.9 volts are referred to herein as galvanic aluminum compositions. Heretofore, no aluminum composition has been known having the capability of producing current galvanically on the continuous basis in the reference cell at a potential more negative than 0.9 volts on the hydrogen scale.

As used herein the term reference cell refers to a galvanic cell such as that described in the Journal of the Electrochemical Society, volume 105, No. 11 (Nov. 1958) starting at page 629. Such a reference cell contains a solution of 0.1 normal sodium chloride in distilled water at 25 C.; and the steel and aluminum electrodes are in the form of rods of square cross section having a total apparent surface area .of 10 square centimeters exposed to the saline solution. The salt bridge used for measuring the potential of the reference couple was located between the electrodes in the reference cell rather than on side, as shown in the cited article, and was connected to a standard calomel electrodeto measure the reference aluminumsteel couple potential. The calomel electrode is connected at about the mid-point of the aluminum-steel reference electrode couple. The abscissa of FIGURE 3 is the potential on the hydrogen scale which is found to exist between the aluminum and the mild steel electrode when the cell is in continuous operation. The ordinate of FIGURE 3 is the number of coulombs found to flow between the reference aluminum-steel electrode couple in 48 hours.

, Generally, the ternary elements which are present in aluminum-tin compositions may be grouped into two categories. The first is the group of impurities which are found in aluminum compositions because of the method used in preparing the composition and because of the association of impurity elements with aluminum in ore. The second group includes ternary elements which are deliberately added to the aluminum-tin composition to modify the properties thereof.

Of the impurity elements, the'two which are the most commonly occurring, are silicon and iron discussed immediately above. Generally, other impurity elements which tend to mask the galvanic properties imparted by tin should be maintained at a concentration below 0.02 percent if high galvanic currents are to be obtained at high efiiciency.

However, it is possible to include percentages of ternary elements in excess of 0.02 percent either where no deleterious effect results from such inclusion or where specific modifications of galvanic behavior are sought by such additions. I

In general, it has now been observedthat the behavior of tenary alloy elements added to aluminum-tin galvanic compositions depends on the solubility of the elements in aluminum and their effect upon the aluminum lattice. More specifically, changes in galvanic behavior of aluminum-tin compositions responsive to the addition of ternary alloy elements to the compositions dependsprim'arily on the eifect of the ternary element on the solubility of tin in aluminum, as it is the tin solute in the aluminum lattice which is primarily responsible for the higher con centration of n-type defects found in aluminum oxide surface layers produced at the surface of the aluminum compositions by reaction with environmental elements.

5 Thus, it has now been found that those ternary addition elements which enter into solid solution and which expand the aluminum lattice, stabilize the tin in retained solid solution and permit high galvanic currents to be drawn from the alloys. Ternary alloying additives which display this behavior include magnesium, zirconium, and bismuth. The use of bismuth as a ternary alloying additive is preferred because it further increases the galvanic current over that obtained from the corresponding aluminum-tin composition from which this particular ternary additive is absent. v

A further group of ternary alloying additives are those which are soluble in aluminum and which cause a contraction of the aluminum lattice. These additives cause a a reduction of the concentration of tin which will remain in solid solution in the alloy and thereby tend to negate the beneficial effect of tin on galvanic properties. Ternary alloying additives which contract the aluminum lattice include copper, zinc, manganese, and silicon.

A third group of ternary alloying additive elements are those which have a very small maximum solubility in aluminum, less than 0.05 percent, and which exist primarily in an insoluble second-phase state. The eifect of ternary alloying additives as second-phase components of the composition is primarily in the reduction in the anodic efliciency. Such second-phase material has only,

a minor effect on galvanic current output of the composition.

The effect of the addition of certain ternary elements of these groups on galvanic properties of aluminum compositions is illustrated in Table I below.

Values of anodic current, anodic efiiciency and couple potential for a reference cell are given in the table for aluminum-tin compositions containing small additions of the ternary alloying elements. The first value listed is of the binary alloy composition aluminum-tin formed by the alloying of 0.12 percent tin with high purity (99.997%) aluminum.

For all other compositions listed the ternary element is alloyed in a composition containing 0.20 percent tin and the balance high purity aluminum.

TABLE I Percent Approximate Anodic Couple Alloying element added number of efliciency, potential coulombs percent Aluminum with 850 to 950 40 -1. 08

0.12% tin I Magnesium 1. 10 800 to 1, 000 54 1. 09 B1smuth 0.16 1, 200 to 1, 600 38 to 66 1. 16 Zirconium. 0. 094 800 50 -1. 09 Zinc 1. 30 67 0. 74 Manganese..- 0. 84 30 52 0. 55

opper 0. 30 59 0. 64 Silver... 0. 090 100 49 0. 90 Do 0. 013 1, 000 48 1. 09 Niekel 0.096 350 55 a 0. 96 Iron 0. 076 825 32 1. Arsenic- 0. 012 700 41 -1. 11 Antimony. 0. 045 400 to 600 46 1. 03 Cobalt 0. 021 600 to 900 38 to 65 1. 07

The galvanic properties of the ternary compositions as given in Table I illustrates the grouping of the ternary additive elements into the three groups as given above based on their effect on the aluminum lattice and on the solubility of tin in aluminum.

Anodic efiiciency High efficiency galvanic aluminum is an aluminum composition from which a given quantity of current can be derived, with a removal from an anode formed of the galvanic aluminum, of less than double the quantity of metal theoretically needed to yield the given current. High efficiency galvanic aluminum is distinct from galvanic aluminurn per se in that the term galvanic aluminum designates aluminum base compositions from which useful high values of current can be derived on a sustained basis at a voltage on the hydrogen scale more negative than I -0.9 volt, without regard to the amount of metal which is consumed or removed from an anode in producing said current in a reference galvanic cell.

Regarding the anodic efficiency of metal compositions containing only tin in solid solution in high purity aluminum, or in aluminum base metal containing less than 0.05 percent silicon and less than 0.1 percent iron, the

anodic efiiciency of such compositions is high and may be in the order of or more for high purity aluminum containing 0.02% tin, for example, when electrically coupled to'a steel cathode both through an external electrical circuit and through an internal saline electrical circuit. However, where an alloy composition is used containing about a maximum of dissolved tin, although the efiiciency is high, this high efiiciency is frequently ac companied by an undesirable pitting. This pitting is an undesirable localized penetration of the metal at a rate greater than that which would result in uniform corrosion of the entire anode. Pitting may ultimately lead to portions of the electrode becoming detached where the electrode is used for an extended term, and to a lowering of the value of ampere hours per pound obtained for the metal of the anode.

It has now been discovered that by addition of secondphase, conducting. local cathodes, numerous corrosion sites can be produced and that through the production of numerous corrosion sites, a desirable uniform corrosion occurs. It has further been discovered that although the addition of the second-phase cathode is accompanied by a loss of etficiency, a sacrificial anode composition can be prepared which has a combination of galvanic properties which are better than those of other metals.

As an additional unexpected advantage of the addition of the conductive secondary tin cathode particles, an increase is found in the amound of current produced at the anode. Thus, where the efiiciency decreased from 70 percent to 58 percent, as the per cent tin increased from 0.10 to 0.125, in high purity aluminum the galvanic current increased quite surprisingly by 83 percent.

Accordingly a second requirement of compositions of the present invention, when put to galvanic uses such as sacrificial anode uses, over and above the requirement for an increased number of n-type defects in the surface layer formed by anionic reactions, is the requirement for the presence of particulate secondary cathodes in the anode. In essence, the second requirement is that the secondary cathode be of such form, composition and distribution within the anode as to cause significant changes in the other factors governing the galvanic properties of the composition.

' Thus, it has been found that where a second phase conductive substance, insoluble both in aluminium base alloy and in the electrolytes, is added to the aluminum-tin anode as a finely divided dispersed particulate material, the second-phase substance acts as a cathode.

There is a definite relationship between the form and quantity of the second-phase cathode particles and the galvanicproperties exhibited by an anode in which they 'are distributed as will be brought out more fully below.

However, in general, either too many or too few of the cathode particles, or particles which are too big or too small, will not give the desired results. 7

It will be appreciated that the second-phase particles become activeas secondary cathodes only as the particles become exposed to the electrolyte environment'at the surface of the anode during the electrolytic dissolution of the anode metal.

The term local action corrosion refers herein to the corrosion which takes place in the vicinity'of particulate tin, or other second-phase cathode particle, at the surface of the sacrificial anode. 7

Because of the intimate contact and close juxaposition of the second-phase cathodes with and to the surface of a sacrificial anode, the anode efficiency is very sensitive tothe quantity and distribution of the second-phase cathode particulate material in the sacrificial anode.

Although it has been discovered that tin is unique among the elecents of Group IV and Group V of the periodic table of elements, in creating a large number of n-type defects in aluminum oxide films and thereby making possible substantial alterations of the galvanic behavior of the aluminum alloy, materials which can'be distributed in the alloy as second-phase cathodes to meet the above requirements for high efiiciency performance may be selected from a wider variety of elements.

In general the material suitable for inclusion as secondphase cathodes in a composition which exhibits improved galvanic properties in accordance with this invention should be capable of being included in said composition in electrical association with the tin-containing aluminum of said composition without appreciably reducing the solid solubility of tin, should preferably be in the form of discrete particles at anode operating temperatures, should have a high electronic conductivity and a .low hydrogen 'overpotential, and should be formed of a substance which is substantially less reactive with the environment than the aluminum of said composition and which does not form high resistance surface layers by said reaction.

As indicated above compositions of the present invention are unique in that they exhibit sustained galvanic activity although prepared with an aluminum metal base. The combination of high anodic efiiciency and high galvanic currents is unique particularly when produced from an aluminum base alloy of essentially commercial purity. Numerous uses can be made of alloys having the compositionsand prepared accordingly to the treatment described herein.

One large scale use of such compositions is as sacrificial anodes in the protection of'metal structures exposed to marine environments. Compositions of essentially commercial purity aluminum base metal having high galvanic outputs and high anodic efliciencies which are suitable for use in aluminum sacrificial anodes, as well as for use in anodes for marine power cells, desirably have a composition containing 0.1 to 0.3% tin, up to 0.05% silicon, up to 0.1% iron and the balance aluminum containing insuflicient amounts of other impurities to materially interfere with the high galvanic output and high anodic efiiciency of said composition.

Regarding the impurity level in galvanic alloy compositions, it is evident fromthe discussion of ternary alloying elements as given above, that numerous elements may be present in said compositions inamounts ineffectual to reduce the galvanic current producing properties ofthe metal. The permissible concentration level of a particular impurity or tenary additive is dependent on the galvanic properties which are to be exhibited by the composition.

It has now been discovered that in addition to the capacity which compositions of the present invention have to be changed with regard to their galvanic charac teristics by heat treatment as discussed below, substantial changes in the galvanic propertiescan be efiected with precision and reliability by compositional changes therein.

particularly to be avoided in preparing alloy compositions for uses which. require high galvanic currents.

For example an ingot of aluminum containing tin and other constituents which would otherwise yield high galvanic currents, gave lowered currents because of the presence of 0.02% zinc as an impurity. Preferably the concentrations of such elements as zinc and copper should be kept at levels below 0.02% to avoid substantial reductions in the maximum solubility of tin in the composition and consequent poor galvanic performance. a For the applications of galvanic aluminum requiring higher galvanic current coupled with high anodic efiiciency the persence of insolubleelements such as nickel, arsenic, antimony, cobalt, and the like, in low concentrations isnot as critical as is the presence of the soluble.

cons'titutents which constnict the aluminum lattice. However, when the concentration of such insolubles is increased this leads to an undesirable reduction in anodic efficiency as described above with reference to the group of insoluble ternary alloy addition elements.

Compositions of aluminum and tin are also useful for the sustained generation of relatively low currents at high efficiencies for use in another group of applications such as in dry cell anodes.

Numerous other applications can be made of the compositions of this invention to make use of their unique galvanic current producing properties. For example, in dry cell applications it may be-desirable to duplicate the electrochemical characteristics of zinc and for such purpose a much lowercurrent output will be desired than the optimum high current obtainable from galvanic aluminum. Pursuant to this invention, such a lower current is achieved by use of a lower tin content. In this regard, galvanic current rises sharply in high purity aluminum compositions containing t-in Where the tin content is increased in the range between 0.04 and 0.08% Sn. By application of thermal treatments, fairly constant current output over a wide compositional '(Sn) range -up to. about 0.1% tin, may be obtained. In order tosimultaneously produce uniform corrosion of the metal specimen, ternary elements may beadded to form secondary cathode inclusions as described above. An alloy composition suitable for producing lower currents at high efiiciency for dry cell utilization may be produced inthis way. Ternary additives, such as Cd. Bi, or Pb may be used for this purpose in that they are metallic conductors, are essentially insoluble in aluminum andexhibit a relatively low hydrogen over-potential. .Ternary additiveswhich are tofserve as a source of secondary cathode. particles are considered esxintially insoluble for this purpose where their solubility does not exceed 0.02%. Alternatively additive elements capable of forming a stable compound withjaluininum such as Fe as (FeAl Cr as (CrAl and Mn as (MnAl may be used for this purpose. These additive elements must be essentially in accordance with the formula for secondary cathode particulate additives as given above and are found to promote the uniform corrosion attributable to the secondary cathode particles.

As a further alternative it has now been discovered that alloy compositions suitable for use in dry cells may be prepared by the addition of a controlled quantity of even slightly soluble elements where these dissolved elements contract the crystal lattice of aluminum and thus serve as moderators of the aluminum-tin compositions containing up to 0.5 percent tin. Such soluble ternary additives serve as moderators in that they change the galvanic behavior of an alloy composition having a high galvanic out-put and high anodic efficienc'y, to one which has a combination of good anodic efiiciency and lowered but stable current outputs. Ternary alloying additive elements which are particularly beneficial as current modifiers are zinc and copper, although any other ternary element which, when added togalvanic aluminum, results in a contraction of the aluminum lattice and a reduction in tin solubility also permits similar controlled reduction of the current outputs. For example, the

areasse use of aluminum-tin compositions, containing 1.0 percent zinc or 0.1 percent copper as moderators, is contemplated for dry cell application as well as those containing 0.1 to 0.3% silicon.

The degree of effectiveness of the addition of a given quantity of a particular ternary additive in changing the galvanic current output which will be obtained from a galvanic aluminum composition is readily determinable by measuring the closed circuit potential of a reference galvanic cell containing the aluminum test specimen as anode. Measurement of this closed circuit potential provides a rapid assessment of the degree of galvanic current output obtainable.

High current outputs suitable for such applications as sacrificial anodes or marine power cell anodes are obtained when the potential between the aluminum and the steel electrode of the reference galvanic cell is more active than 0.9 volt on the hydrogen scale. In general, anode compositions which when thus tested as a couple are found to have more active, or correspondingly less noble, potential, are also more active galvanically and yield higher galvanic currents as illustrated in FIGURE 3. On the other hand, if the measured potential of a particular test couple in the reference galvanic cell is more noble than 0.9 volt on the hydrogen scale, then the galvanic activity of the alloy composition used in forming the test galvanic couple is generally too limited for compositions containing galvanically activating amounts 1 of tin where the reference cell galvanic test couple potential is more noble than 0.9 volt when these compositions have uniformly distributed therein the unreactive conductive particles to adapt such compositions to the high eil'iciency current production attainable in accordance with this invention by provision of secondary cathode particles substantially in accordance with the above equation.

As pointed out in the earlier applications S.N. 60,166, the desirable galvanic properties of such compositions are due in part to the presence of tin therein in a form which yields n-type defects in surface coatings formed as reaction products of said composition with environmental reactants such as atmospheric oxygen or sea water, or with other reactants which yield aluminum oxide in the surface coating. For maximum galvanic currents at maximum of tin must be present as supersaturated solid solute. Generally, maximum solute tin concentration and maximum galvanic currents are achieved reproducibly and reliably by a homogenization treatment.

The term homogenization treatment is intended to mean thermal treatment of an aluminum tin alloy which will maximize the uniform distribution of tin in the alloy and will provide specimens having the maximum amount of retained tin in a metastable solid solution, i.e., about 0.1 percent. Such a treatment can be accomplished by a soaking of the composition containing aluminum and tin for 16 hours at 620 C. Unless otherwise specified, com- Generally, there is less tendency for aluminum-tin compositions of lower purity to exhibit optimum galvanic properties in the as cast condition. For such compositions the use of homogenization treatment to solubilize the tin is effectively essential.

In addition to the homogenization treatment referred to above, the purpose of which is to put a maximum of tin into solid solution, where it is desired to partially precipitate the dissolved tin, a heterogenizing treatment is employed. The term heterogenizing, heterogenized alloy, and the like refers to a treatment of aluminum-tin composition in which tin is in metastable solution at a given concentration level, to lower the concentration level and to transform at least a portion of the tin solute into a second phase state.- A reference heterogenization treatment involved 24 hours heating at 400 C. followed by a water quench.

Although heterogenizing may be carried out at other temperatures, for example 300 or 500 C., and although longer and shorter times of heating may be employed (generally in inverse relation to the temperature of heating), the above combination of temperature and time is effective in removing much tin from solid solution and is the procedure referred to by the term heterogenizing as used hereinafter unless some indication. is given to the contrary. There is nodetectible loss of tin from use of this heterogenizing treatment.

This heterogenizing treatment is efiective in eliminating the beneficial effects of tin on galvanic corrosion characteristics only up to 0.08%. At a tin of 0.12%, galvanic currents are slightly reduced by this treatment and at a content of 0.20%, no significant efiect is produced. These results furth evidence the great stability of the galvanic corrosion characteristics in the range of 0.1- to 0.2% tin and that the decomposition of the metastable solid solution on long term aging at room temperature will not occur.

One of the advantages of the aluminum-tin prepared in accordance with thi invention by use of the homogenizing step is the high degree of stability exhibited on aging during periods of up to one year at room temperature. No loss of the superior galvanic activity of such compositions is apparent after such long aging.

From the foregoing it is evident that a unique group of aluminum alloy compositions and articles are taught which are adapted to providing a variety of galvanic properties based on control of the tin and ternary element concentration therein and of the thermal treatment thereof. Several compositions have now been discovered, the galvanic properties of which are also dependent on the presence of n-type defects in the reaction-product surface layers and of secondary cathodes of pecific requirements, which are similarlyresponsive to thermal treatment and which are capable of producing galvanic current at a high rate and high potential at low cost.

In particular compositions and articles which can supply useful high galvanic currents at a higher rate and at a lower cost per unit of electrical energy produced than possible with any galvanically active alloy known heretofore are formed in accordance with this invention. The

low cost of these galvanic aluminum alloys is attributed to their being formed with an aluminum base of essentially commercial purity aluminum, and to the reliability and reproducibility of the galvanic properties of the alloys produced due to their responsiveness to thermal treatment, and compositional modifications.

Since many examples of the foregoing compositions and articles may be carried out and made, and since many modifications can be made in the articles and compositions 7' described without departing from the scope of the subject invention, the foregoing is to be interpreted as illustrative only, and not as defining orlimiting the scope of the invention.

What is claimed is the following: 1. An aluminum base alloy consisting essentially of from 0.04 to 0.5 percent tin, with said tin being present 2. An alloy according to claim 1 wherein said tin is present in an amount of from 0.1 to 0.5 percent.

3. -A composition according to claim 1 wherein said tin is present in an amount of from 0.1 to 0.3 percent.

4. An aluminum base alloy consisting essentially of from 0.04 to 0.5 percent tin, with said tin being present in solid solution to the maximum degree at room temperature, said maximum degree being 0.1 percent, an effective amount of an element which enters into solid solution in said aluminum and Whichexpands the aluminum lattice, and also containing silicon in the amount less than 0.05 percent and iron in an amount less than 0.1 percent and the remainder aluminum.

5. A composition according to claim 4 wherein said element which enters into solid solution is selected from the group consisting of magnesium, zirconium, and bismuth.

6. An alloy according to claim 4 wherein said element which enters into solid solution is magnesium present in an amount of about 1.1 percent.

7. An alloy according to claim 4 wherein said element which enters into solid solution is bismuth present in the amount of about 0.16 percent. a

8. An alloy according to claim 4 wherein said element which enters into solid solution is zirconium present in an amount of about 0.094 percent.

9. An aluminum base alloy consisting essentially of from 0.04 to 0.5 percent tin, with the tin being present in solid solution to a maximum degree at room temperature,

said maximum degree being 0.1 percent, said alloy also containing silicon in an amount less than 0.05 percent, iron in an amount lesst-han 0.1 percent, zinc in an amount less-than 0.02 percent, and copper in an amount less than 0.02 percent and the remainder aluminum.

10. An aluminum base alloy consisting essentially of from 0.04 .to 0.5 percent tin, with the tin being present in solid solution to the maximum degree at room temperature, said maximum degree being 0.1 percent, said alloy also containing silicon in an amount less than 0.05 percent, iron in an amount less than 0.1 percent and a small amount of at least one compound having a maximum solubility in aluminum of less than 0.02 percent and the remainder aluminum.

11. An alloy according to claim 10 wherein said compound having a solubility less than 0.02 percent is selected from the group consisting of nickel, arsenic, antimony, and cobalt.

' References Cited by the Examiner UNITED STATES PATENTS 1,997,165 4/35 Brown 204148 2,796,456 6/57 Stokes 136-100 2,820,693 1/58 Hevert et a1. 204-148 7 2,874,079 2/59 .Lozier et a1 13610O 2,913,384 11/59 Staley 104 -148 3,063,832 11/62 Snyder 75-138 FOREIGN PATENTS 636,433 4/50 Great Britain.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner. 

1. AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF FROM 0.04 TO 0.5 PERCENT TIN, WITH SAID TIN BEING PRESENT IN SOLID SOLUTION TO THE MAXIMUM DEGREE AT ROOM TEMPERATURE, SAID MAXIMUM DEGREE BEING 0.1 PERCENT, SAID ALLOY ALSO CONTAINING SILICON IN AN AMOUNT LESS THAN 0.05 PERCENT AND IRON IN AN AMOUNT LESS THAN 0.1 PERCENT AND THE REMAINDER ALUMINUM. 